1. Field of the Invention
The present invention relates to an electrostatic ultrasonic transducer drive control method, an electrostatic ultrasonic transducer, an ultrasonic speaker using the same, an audio signal reproduction method, an ultra-directional acoustic system, and a display device capable of generating constant high sound pressure throughout a broad frequency range.
The present invention claims priority based on Japanese Patent Applications JP 2005-364371 filed on Dec. 19, 2005, and JP 2006-318700 filed Nov. 27, 2006, the contents of which being incorporated herein by reference.
2. Background Art
In the past, most ultrasonic transducers have been of resonance type using piezoelectric ceramic.
Here, FIG. 15 shows a configuration of such an ultrasonic transducer in the past. In the past, most ultrasonic transducers have been of resonance-type using piezoelectric ceramic as a vibrator element. The ultrasonic transducer shown in FIG. 15 performs both conversion from an electric signal to an ultrasonic wave and conversion from an ultrasonic wave to an electric signal (transmission and reception of an ultrasonic wave) using piezoelectric ceramic as the vibrator element. The bimorph-type ultrasonic transducer shown in FIG. 15 is composed of two piezoelectric ceramics 61, 62, a cone 63, a case 64, leads 65, 66, and a screen 67.
The piezoelectric ceramics 61, 62 are bonded with each other, and leads 65 and 66 are connected to the opposite sides to the bonded surfaces thereof, respectively.
Since the resonance-type ultrasonic transducer utilizes resonance of the piezoelectric ceramics, a preferable characteristic of transmitting and receiving the ultrasonic wave is obtained in a relatively narrow frequency band around the resonance frequency.
In contrast to the resonance-type ultrasonic transducer, electrostatic ultrasonic transducers have been known in the past as wide band oscillation-type ultrasonic transducers capable of generating high sound pressure throughout the high-frequency band. These electrostatic ultrasonic transducers are called Pull-type because the vibration films work only in the direction in which the vibration films are pulled towards fixed electrodes. FIG. 16 shows a specific configuration of a wide band oscillation-type ultrasonic transducer (Pull-type). The electrostatic ultrasonic transducer shown in FIG. 16 uses a dielectric member 131 (an insulation member) such as polyethylene terephthalate resin (PET) with a thickness of about 3 through 10 μm as the vibration member. The dielectric member 131 is provided with an upper electrode 132, which is formed as a metal foil such as aluminum, integrally formed on the upper surface thereof by, for example, vapor deposition, and with a lower electrode 133 so as to be contiguous with the lower surface of the dielectric member 131 made of. The lower electrode 133 is provided with a lead 152 connected thereto, and is fixed to a base plate 135 made of, for example, bakelite.
Further, the upper electrode 132 is provided with a lead 153 connected thereto, and the lead 153 is connected to the direct current bias power supply 150. It is arranged that the direct current bias power supply 150 continuously applies a direct current bias voltage of about 50 through 150V for absorbing the upper electrode to the upper electrode 132 so that the lower electrode 133 absorbs the upper electrode 132. The reference numeral 151 denotes a signal source.
A case 130 swages the dielectric member 131, the upper electrode 132, and the base 135 with metal rings 136, 137, 138, and a mesh 139.
A surface of the lower electrode 133 facing the dielectric member 131 is provided with a plurality of microscopic grooves of about several tens through several hundreds of micrometers having uneven shapes formed thereon. These microscopic grooves form gaps between the lower electrode 133 and the dielectric member 131, and accordingly, the distribution of the capacitance between the upper electrode 132 and the lower electrode 133 has a slight variation. These microscopic random grooves are formed by roughening the surface of the lower electrode 133 with a file by manual procedures. In electrostatic ultrasonic transducers, by thus forming an indefinitely large number of capacitors with gaps having different sizes or depths, the frequency characteristic of the ultrasonic transducer shown in FIG. 16 becomes of a wide band as illustrated with the curve Q1 in FIG. 17.
In the ultrasonic transducer having the above configuration, it is configured that a rectangular wave signal (50 through 150Vp-p) is applied between the upper electrode 132 and the lower electrode 133 in the condition in which the direct current bias voltage is applied to the upper electrode 132. It should be noted that as illustrated with the curve Q2 in FIG. 17, the frequency characteristic of the resonance-type ultrasonic transducer has the central frequency (the resonance frequency of piezoelectric ceramic) of, for example, 40 kHz, and the sound pressure of −30 dB from the maximum sound pressure in a frequency range of ±5 kHz with respect to the central frequency, which corresponds to the maximum sound pressure.
In contrast, the frequency characteristic of the wide band oscillation-type ultrasonic transducer having the above configuration is flat in a range from 40 kHz to nearly 100 kHz, and has the sound pressure of about ±6 dB in 100 kHz with respect to the maximum pressure (see Patent Documents 1, 2).
[Patent Document 1] JP-A-2000-50387
[Patent Document 2] JP-A-2000-50392
As described above, in contrast to the resonance-type ultrasonic transducer shown in FIG. 15, the electrostatic ultrasonic transducer shown in FIG. 16 has been known in the past as a wide band ultrasonic transducer (Pull-type) capable of generating relatively high sound pressure throughout a wide frequency band. However, as shown in FIG. 17, the maximum value of the sound pressure, of the resonance-type ultrasonic transducer is 130 dB or more whereas that of the electrostatic ultrasonic transducer is as low as 120 dB, which is slightly insufficient for utilizing the transducer as an ultrasonic speaker.
Here, explanations regarding the ultrasonic speaker will be presented. It tends to mean that AM modulation is executed on a signal in the ultrasonic frequency band called carrier wave in accordance with an audio signal (a signal in the audio frequency band) to drive the ultrasonic transducer with the modulated signal, thus an acoustic wave in the state in which the ultrasonic wave is modulated with the audio signal of a signal source is emitted in the air, and by nonlinearity of the air, the original audio signal is self-reproduced in the air.
More specifically, the principle is that since acoustic waves are compressional waves transmitted by the medium of air, dense portions and nondense portions dominantly appear in the air in the process of transmitting modulated ultrasonic waves, and since the velocity of sound is high in the dense portions and low in the nondense portions, distortion is generated in the modulated wave itself, and as a result, waveform separation into carrier waves (ultrasonic waves) and audible sound waves (original audio signals) occurs, thus we humans can only hear the audible sound (the original signals) with a frequency range of no higher than 20 kHz, which is generally called a parametric array effect.
Although the ultrasonic sound pressure no lower than 120 dB is required in order for sufficiently exerting the parametric effect described above, it is difficult for electrostatic ultrasonic transducers to achieve this numerical value, and accordingly, ceramic piezoelectric elements such as PZT or polymer piezoelectric elements such as PVDF have been mainly used as ultrasonic emitters.
However, piezoelectric elements each have an acute resonance point irrespective of the material thereof, and are put into practical use as the ultrasonic speakers by driving them in the resonance frequencies, and accordingly, the frequency ranges in which the high sound pressure is assured are extremely narrow. It can be said that they are narrow-band.
In general, the maximum audible frequency band of the human ears is said to be from 20 Hz to 20 kHz, and has a bandwidth of about 20 kHz. In other words, in the ultrasonic speakers, it is prevented to faithfully demodulate the original audio signal if the high sound pressure is not assured throughout the 20 kHz frequency band in the ultrasonic wave region. It will be easily understood that it is difficult to perform faithful reproduction (demodulation) in such a wide band as 20 kHz by the resonance-type ultrasonic speakers using the piezoelectric elements of the related art.
In fact, in the ultrasonic speakers using the resonance-type ultrasonic transducers of the related art, the following problems have arisen. 1. The narrow frequency band degrades the reproduced sound quality. 2. The modulation depth is limited to as large as about 0.5 because the demodulated sound is distorted with too large AM modulation depth. 3. If the input voltage is raised (the volume is turned up), the vibration of the piezoelectric element becomes unstable to cause the sound to get distorted, and with further raised voltage, the piezoelectric element itself might be damaged easily. 4. It is difficult to be formed as an array, with a large scale, or with a small size, and accordingly, the cost thereof is high.
In contrast, the ultrasonic speakers using the electrostatic ultrasonic transducers (Pull-type) shown in FIG. 16 can solve almost all problems the above technology of the related art has, but in turn has a problem that the absolute sound pressure is not sufficient for obtaining a sufficient sound volume of the demodulated sound although the wide band can be covered.
Further, since in the Pull-type ultrasonic transducers, the electrostatic force acts only in the direction for pulling the vibration films towards the fixed electrode side, and accordingly, the symmetric property in vibration of the vibration films (corresponding to the upper electrode 132 in FIG. 16) is not maintained, in the case in which the ultrasonic transducers are used for the ultrasonic speakers, there is a problem that the vibration of the vibration films directly cause audible sound.
In this regard, we have already proposed an ultrasonic transducer capable of generating an acoustic signal with a sufficiently high sound pressure level for obtaining the parametric array effect throughout a wide frequency band. This ultrasonic transducer is configured to hold a vibration film having a conductive layer between the a pair of fixed electrodes provided with through holes in the corresponding positions, and to apply an alternating-current signal to the pair of fixed electrodes in the condition in which the direct current bias voltage is applied to the vibration film.
This ultrasonic transducer, which is called a Push-Pull-type ultrasonic transducer, can not only provide sufficiently large vibration of the vibration film for obtaining the parametric array effect because the electrostatic attractive force and the electrostatic repulsive force act on the vibration film held between the pair of fixed electrodes simultaneously in the same directions according to the polarity of the alternating-current signal, but also generate higher sound pressure compared to the Pull-type ultrasonic transducer in the related art throughout the wide frequency band because the symmetric property of vibration is assured.
However, since the Push-Pull type ultrasonic transducer has the through holes, through which the sound passes, with the relatively small areas, it is problematically difficult for the Push-Pull-type ultrasonic transducer as it is to generate sufficient sound pressure in the air.
Therefore, even in the Push-Pull-type ultrasonic transducer having such a configuration, a technology for generating sufficient sound pressure is also required.
Further, if the high sound pressure is generated throughout a wide frequency range, an added value as an ultrasonic transducer increases.